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Odd... I stored them all at 38 PSI, and three have dropped to 34 PSI over 6 months but one dropped to 20 PSI. They were stacked on top of each other, with cardboard in between each one.

I didn't note if the one with the lowest pressure was the one on the bottom of the pile, in case the extra weight on it caused it to lose more air than the others, though you'd think that it wouldn't have as big of an impact as above (34 vs. 20)? If it was related to it, you'd expect the second tire on the bottom to have lost more air too, since it would have two on top of it, vs. the last one that had three on top...

So besides a small leak in it somewhere, anything other suggestions what it could be? Or is it normal that different tires sometimes lose different amounts of air over 6 months, and it's within what could be considered normal? I don't recall the tire losing much air before I took them off last year and I measure pressures monthly, so I'm baffled how it could be a leak/puncture...

I'll be monitoring it in a week vs. the others to see if it's still losing more air or not.

Since I already put them on the car (wasn't about to put the summers back on after having taken them all off!), I'm going to see in a few days if the drop in that one tire corresponds to the other one or not, and since it's under load even now that they're mounted, if there's a leak, it should then be really evident I'm thinking.

I'm leaning towards nothing is wrong though, since at the end of last season when I took them off, I hadn't noticed any pressure drops, hence I was wondering if my theory of "some tires just lose more air than others when stored" could be true... Though you'd think if that was the case, there's a small manufacturing defect or something in it, because it shouldn't be this dramatic...

it could be the seating area on the wheel with corrosion or mis-seat that is causing a very small air loss.

This is what I would guess. That or the valve stem has a very small leak. It's unlikely the tire would have a puncture.

This has happened to me on multiple occasions and generally the low tire will hold air and just leak out very slowly. I had to fill my tire once every week or two before I made it in to Discount Tire and had them fix it.